Varnish impregnation method and apparatus

ABSTRACT

Apparatus is provided for carrying out a varnish impregnation method wherein a sheet-like base material is sequentially passed through a low-viscosity liquid storage region storing a low-viscosity liquid such as a solvent, a thermal syphon region, and a varnish storage region storing a varnish, the thermal syphon region being positioned between the low-viscosity liquid storage region and varnish storage region and communicating with both regions at air tight seals closed by liquid surfaces, and heating the base material in the thermal syphon region to vaporize the low-viscosity liquid impregnated in the base material. When the base material passes through the low-viscosity liquid, air contained in the base material is replaced with the low-viscosity liquid and discharged outside the apparatus. The low-viscosity liquid impregnated in the material is vaporized in the thermal syphon region. The base material containing only vapor of the low-viscosity liquid is transferred to the varnish storage region without being in contact with the atmosphere. The thermal syphon region has a base material inlet opening under or above the surface of the liquid in the low-viscosity liquid storage tank and a base material outlet opening under the surface of the liquid in the varnish storage tank. The thermal syphon region has a heating roll therein for heating the base material to thereby evaporate low-viscosity liquid impregnated in the base material.

FIELD OF THE INVENTION

The present invention relates to a varnish impregnation process andapparatus for sheet-like fibrous base materials such as paper or clothto be used in the manufacture of laminated boards such as facing boards.

PRIOR ART

In effectively impregnating any sheet-like fibrous base material withvarnish, it is essential not only for the varnish to be absorbed intoand distributed evenly throughout the fibrous base material, but alsofor air bubbles contained in the base material to be eliminated to thegreatest possible extent.

According to conventional varnish impregnation processes, however, it iscommon practice to impregnate the base material with ordinary varnish ata preliminary impregnation step, and then the base material isintroduced by a timing roll into a varnish tank to be impregnated withthe varnish. Such a conventional impregnation process, however, is knownto have problems in that the varnish is not absorbed evenly andadequately into the base material, and the varnish takes too long to beabsorbed. Such problems are especially conspicuous where ahigh-viscosity varnish is used.

Recently, however, an improved impregnation process has been developedutilizing a preliminary impregnation tank containing a solvent or a thinvarnish containing a large amount of solvent (hereinafter referred to as"preliminary impregnation liquid") and a regular impregnation tankcontaining regular varnish. The base material is passed through thepreliminary impregnation liquid tank and the regular impregnation tankcontaining the regular varnish by being guided with guiding rollers.

According to this impregnation process, air contained in the basematerial is removed by being replaced with the preliminary impregnationliquid while passing through the preliminary impregnation liquid, sothat the varnish can be made to be absorbed effectively. The air bubblescan be reduced if the base material impregnated with the preliminaryimpregnation liquid is brought into the regular varnish while permittingthe preliminary impregnation liquid to evaporate.

In the improved impregnation process, however, the base material is notonly exposed to the air but also subjected to the pressure of the guiderolls while moving from the preliminary impregnation liquid tank to thevarnish tank, so that neither satisfactory impregnation of the varnishnor the absence of air pockets can be expected. More particularly, whenthe base material is brought onto a guide roll, the contact surfacepressure acts on the fiber bundles constituting the base material toopen the fiber bundles, thereby destroying the liquid phase of thepreliminary impregnation liquid maintained by capillary action in thefiber bundles. The fiber bundles are released from the compressive forceacting thereon as the base material passes the guide rollers, and thefiber bundles, opened due to the compressive pressure, are formed intobundles again. Since destruction of the impregnation liquid phase andrestoration of the fiber bundles take place in air, air re-enters thebase material while the base material moves into the varnish from thepreliminary impregnation liquid.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a varnish impregnationprocess and apparatus which permits even and adequate impregnation of abase material with varnish, the impregnated base material having no airbubbles therein, thus ensuring high quality finished products such aselectric insulating boards or facing boards in which the impregnatedbase material is used.

Another object of the present invention is to provide a varnishimpregnation process and apparatus which enables a substantial reductionin the dimensions, and substantial simplification of the constructionof, the apparatus without using a timing roll or the like.

Another object of the present invention is to provide a varnishimpregnation process and apparatus which enables the quantity oflow-viscosity liquid used in the base material to be reduced to anextremely small amount. This permits a substantial reduction in theamount of vapor of the low-viscosity liquid exhausted from a dryingfurnace in a drying step following the impregnation process.

Another object of the present invention is to provide a varnishimpregnation process and apparatus which enables the loss of thelow-viscosity liquid and the thinning of the varnish to be effectivelyprevented and also enables the impregnation of a base material withvarnish to be facilitated.

Another object of the present invention is to provide a varnishimpregnation process and apparatus which enables the pressure of thevapor in a thermal syphon to be kept within a certain range irrespectiveof alterations or changes in the varnish impregnation conditions, suchas base material feed speed, to insure satisfactory impregnation of abase material with a varnish.

Another object of the present invention is to provide a varnishimpregnation process and apparatus which enables the vapor pressure in athermal syphon region to be controlled more surely and accurately thanthe case where a pressure control device is employed, not only withoutusing any high precision pressure control device at all, but alsowithout requiring any further sophistication and extension of the deviceand its construction.

Another object of the present invention is to provide a process andapparatus which prevents undesirable conditions such as thelow-viscosity liquid and the varnish being drawn into a thermal syphonroom even when a vacuum or negative pressure is formed in the thermalsyphon room.

In a varnish impregnation process and apparatus according to the presentinvention, a thermal syphon region, communicating with a varnish storageregion in a liquid-sealed condition with the surface of the varnish, isprovided between a low-viscosity liquid storage region containing alow-viscosity liquid such as a solvent, and a varnish storage regioncontaining a varnish arranged such that a sheet-like base material maybe passed through the low-viscosity liquid storage region, the thermalsyphon region and the varnish storage region sequentially and the basematerial can be heated in the thermal syphon region to enable thelow-viscosity liquid, absorbed in the base material, to be evaporated.

Within a region extending from the base material heater in the thermalsyphon region to the varnish storage region, the base material is heatedto, or kept at, a temperature that is higher than the temperature atwhich the vapor of the low-viscosity liquid starts to condense. Thevapor of the low-viscosity liquid, produced in the thermal syphonregion, is condensed and collected in liquid form. The thermal syphonregion may communicate with the low-viscosity liquid storage region toform a liquid-seal at the surface of the liquid so that thelow-viscosity liquid in the communicating region may be heated forevaporation, and the rate of evaporation can be controlled according tochanges in the liquid-sealed level in the communicating region. In thethermal syphon region, it is desirable to enable the vapor of thelow-viscosity liquid generated in said region to flow from the side ofthe base material outlet towards the inlet for the base material. Thecommunicating region between the thermal syphon region and the varnishstorage region, may be heated to, and kept at, a level that is higherthan the temperature at which the low-viscosity liquid starts to boil.

Apparatus for practicing the above-mentioned process comprises alow-viscosity liquid storage tank containing a low-viscosity liquid suchas a solvent, a varnish storage tank containing a varnish, a thermalsyphon room with a base material inlet opening under or above thesurface of the liquid in the low-viscosity liquid storage tank and abase material outlet opening under the surface of the liquid in thevarnish storage tank, a guiding mechanism for enabling a sheet-like basematerial consisting of a fibrous material to pass sequentially throughthe low-viscosity storage tank, the thermal syphon room and the varnishstorage tank, and a heater to heat the base material to evaporate thelow-viscosity liquid absorbed in the base material.

The apparatus may be provided with a cooler to liquefy the vapor of thelow-viscosity liquid by condensing and a liquid collector to collect theresulting liquid. Further, the apparatus may be provided with acondensation prevention means for heating to, or keeping the basematerial at, a temperature higher than that at which vapor of thelow-viscosity liquid starts to condense. The condensation preventionmeans extends from a base material heater in the thermal syphon room tothe end of an opening in the base material outlet and surrounds the pathof the base material. Where the base material inlet is made to openunder the surface of the liquid in the low-viscosity liquid storagetank, it is desirable to provide a low-viscosity liquid heating surfaceextending in the vertical direction and located in the base materialinlet so that the heating surface will be able to function as alow-viscosity liquid heating mechanism which enables the evaporationrate of the low-viscosity liquid due to heating to be varied accordingto changes in the level of the liquid surface in the low-viscosityliquid storage region. Further, it is desirable to provide alow-viscosity liquid storage region and a low-viscosity liquid heater atthe base material outlet, and it is also desirable to provide a vaporflowing mechanism which enables not only the low-viscosity liquid to beheated for evaporation, but also enables the vapor of the low-viscosityliquid generated in the thermal syphon room to flow from the side of thebase material outlet towards the base material inlet. In this case, thelow-viscosity liquid storage tank and the low-viscosity liquid storageregion should communicate with each other. Further, it is desirable forthe vapor flowage mechanism to be provided with a low-viscosity liquidcollector communicating with the low-viscosity liquid storage regionlocated at the base material inlet, and a cooler located at thelow-viscosity liquid collector. In some embodiments the wall surroundingthe thermal syphon room may have a jacket construction so that a heatingmedium can be supplied and flowed through the jacket. A heating devicemay be provided to heat the boundary of the varnish surface in the basematerial outlet to a temperature higher than the boiling pointtemperature of the low-viscosity liquid. The cross-sectional area of thebase material passage in the base material outlet should be as small aspossible while still permitting passage of the base material. A varnishimpregnation quantity control device is provided to adjust the quantityof varnish contained in the base material after it has passed throughthe varnish storage tank. A vapor injection nozzle to supply vapor ofthe low-viscosity liquid into the thermal syphon room may be provided.In some embodiments the top of the low-viscosity liquid storage tank maybe closed with a cover provided with a base material inlet, and the basematerial inlet may be provided with a cooler to prevent leakage of thevapor.

Other objects, features, aspects and advantages of the present inventionwill become apparent upon consideration of the following detaileddescription of the invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a varnish impregnation apparatusaccording to a first embodiment of the invention;

FIGS. 2A and 2B illustrate low-viscosity liquid levels resulting fromchanges in vapor pressure in the thermal syphon room;

FIG. 3 is a temperature vs. vapor pressure diagram of methyl ethylketone, used as a low-viscosity liquid;

FIG. 4 is a sectional view showing a varnish impregnation apparatusaccording to a second embodiment of the invention;

FIG. 5 is a sectional view showing a varnish impregnation apparatusaccording to a third embodiment of the invention;

FIG. 6 is a sectional view of a base material outlet extending below thelevel of varnish in a varnish tank;

FIG. 7 is a sectional view of a varnish impregnation apparatus whereinthe thermal syphon room is provided with a jacket construction;

FIG. 8 is a sectional view of a varnish impregnation apparatus accordingto a fourth embodiment of the invention;

FIG. 9 is a sectional view of a varnish impregnation apparatus having avapor cooler at the base material inlet to the thermal syphon room;

FIG. 10 is a sectional view of a varnish impregnation apparatus whereinthe storage tank for storing the low-viscosity liquid is provided with acover and a cooler at a base material inlet to the storage tank;

FIG. 11 is a sectional view of an impregnation apparatus having a coolerfor condensing the vapor of the low-viscosity liquid into a collectorwhich communicates with a low-viscosity liquid storage section;

FIG. 12 is a sectional view of a fifth embodiment of a varnishimpregnation system according to the present invention;

FIGS. 13 and 14 are side views of variations of the varnish impregnationprocess according to the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates a first embodiment of the invention wherein a varnishimpregnation system comprises an open-top low-viscosity liquid storagetank 2 containing a predetermined quantity of low-viscosity liquid 12such as a solvent, an open-top varnish storage tank 3 containing apredetermined quantity of varnish 13, a thermal syphon room 4 locatedbetween the two storage tanks 2 and 3, a base material heater 5 to heatthe base material 1 in the thermal syphon room 4, a cooling device 6 tocondense and liquefy the vapor of the low-viscosity liquid, a guidingdevice 7 to guide or transport the run of the base material 1 along apredetermined course, a condensate collector 8 to collect the condensateof the low-viscosity liquid 12 and a varnish impregnation control device9.

The base material 1 is in a sheet-like form and comprises fibrousmaterials such as woven cloth or non-woven cloth made of synthetic ornatural and organic or inorganic fibers.

The low-viscosity liquid 12 should have an adequate wettability to thebase material 1. For example, the solvent should be one whose viscosityis lower than that of the varnish 13 or lower than 100cP, but such asolvent should preferably have the same properties as those of thesolvent mixed with the varnish so that there will be no problem evenwhen the solvent is mixed with the varnish 13. The low-viscosity liquid12 is kept at a predetermined temperature Ts by a temperature controllerand heater (not shown) of conventional design.

It is preferred that the varnish 13, be a thermosetting resin varnishalthough other varnishes, such as thermoplastic resin and naturalresins, or liquid synthetic resins and liquid natural resins notcontaining the solvent may also be used. A temperature controlled heater(not shown) keeps varnish 13 at a predetermined temperature Tw that ishigher than the temperature Ts of the low-viscosity liquid.

The thermal syphon room 4 is disposed above the storage tanks 2 and 3and formed into a reverse U-shaped syphon tube with an inlet 4a throughwhich the base material enters from storage tank 2 and an outlet 4bthrough which the base material exits into storage tank 3. The inlet 4aand the outlet 4b open under the liquid surface 12' of the low-viscosityliquid storage region 2' and the liquid surface 13' of the varnishstorage region 3', respectively, to form a liquid-sealed thermal syphonregion 4' in the thermal syphon room 4. The base material inlet 4a has arelatively large cross-sectional area to secure a large condensationarea for the saturated vapor of the low-viscosity liquid, whereas thebase material outlet 4b has a relatively small cross-sectional area toprovide a relatively small evaporation area for the saturated vapor ofthe low-viscosity liquid. The wall surrounding the thermal syphon room4, including the base material inlet 4a and the base material outlet 4b,comprises a heat insulator so as to prevent the vapor of thelow-viscosity liquid from being cooled by outside air, thus reducingcondensation of the vapor on the internal surface of the wall.

The thermal syphon room 4 is connected to an exhaust pump 15 thorugh ableeding tube 14 and a pressure regulating valve 16. This not onlypermits air in the thermal syphon region 4' to be eliminated but alsopermits the pressure of the region 4' to be controlled. The pump 15 isconnected to a separator 17 so that the vapor 12s of the low-viscosityliquid, contained in the air exhausted through bleeding tube 14, can beseparated and collected in the form of the saturated liquid 12 andreturned to the low-viscosity liquid storage tank 2 through a collectingtube 18.

The base material heating device 5 comprises a heating roll 5a installedrotatably in the thermal syphon room 4 so as to be able to follow thecircular run of the base material 1. In the thermal syphon region 4',the heating roll 5a heats the base material 1 to a temperature higherthan, or approximating the boiling point temperature of thelow-viscosity liquid 12, to evaporate the low-viscosity liquid 12absorbed in the base material. The heating roll 5a may be driven torotate in the direction of movement of the base material 1. If theheating roll is driven, it is necessary to provide suitable means foradjusting its circumferential speed to the speed of the base material 1.

The temperature Th (<Tw) to which the base material is heated by theheating roll 5a is controlled by a temperature controller (not shown).The evaporating ability of the heating roll should be large enough toevaporate from the base material 1 the largest quantity of low-viscosityliquid brought into thermal syphon room 4 by the base material.

The cooling device 6 comprises a cooling coil 6a located at the top endof the base material outlet 4b in the thermal syphon room 4. The coolingcoil 6a liquefies or condenses the vapor 12s of the low-viscosity liquidproduced in the thermal syphon region 4' to provide the effectivethermal syphon action. The cooling coil also prevents, to the largestextent possible, condensation of vapor 12s in the varnish storage region3'. The cooling coil is maintained at a temperature tc which is lowerthan the temperature Ts of the low-viscosity liquid.

The guiding device 7 comprises a plurality of guide rolls 7a and 7binstalled above low-viscosity liquid storage tank 2 and in the storagetanks 2 and 3 so that the base material 1 from a base material supplysource 1' can be guided through the low-viscosity storage region 2' intothe thermal syphon region 4' where it passes over heating roll 5a. Thebase material is then guided through cooling coil 6a, base materialoutlet 4b and varnish storage region 3' from whence it passes throughthe varnish impregnation control device 9. At least one of the guiderolls 7a and 7b disposed in each of the storage regions 2' and 3' is ofthe expander type so that an expanding force can be applied in thelateral direction, orthogonal to the direction of advancement of thebase material 1. This facilitates replacement of air in the basematerial with the low-viscosity liquid in the storage region 2' andimpregnation of the base material with varnish in the varnish storageregion 3'.

The condensate collection device 8 comprises a condensate pool 8a formedat the bottom of the thermal syphon room 4 and immediately below thecooling coil 6a, and a downwardly inclined member 8b for down flow ofthe condensate into storage tank 2.

The varnish impregnation control device 9 comprises a squeeze roll or asqueeze bar disposed above storage tank 3 for squeezing excess varnishfrom the base material as it emerges from the varnish storage tank 3.

The varnish impregnation system described above operates as follows tocarry out the process according to the present invention. Firstly, thebase material 1 from the base material supply source 1' is brought intothe storage tank 2 where it is immersed in the low-viscosity liquid 12,so that air in the base material 1 is replaced with the low-viscosityliquid. That is, when the base material 1 reaches the surface 12' of thelow-viscosity liquid 12, the low viscosity liquid 12 permeates into thefiber bundles 1a (FIG. 2A) of the base material due to capillary action.Simultaneously, air in the fiber bundles is forced out due to theosmotic pressure of the liquid. Such an osmotic action, however, stopsat a certain level due to the osmotic resistance of the fiber bundles1a. The stop of the osmotic action occurs above or below the liquidsurface 12' depending on whether the traveling speed (V₁) at which thefiber bundles 1a enter the low-viscosity liquid 12 is larger or smallerthan the osmotic speed V₂. FIG. 2A illustrates the condition where V₁>V₂ while FIG. 2B illustrates the condition where V₁ <V₂.

As the base material 1 passes through the low-viscosity liquid storageregion 2', all the air in the fiber bundles 1a is replaced with thelow-viscosity liquid 12. The expander type guide roll 7a facilitates thereplacement by forcing the air from the fiber bundles.

Next, the base material 1, impregnated with the low-viscosity liquid 12,is fed into the thermal syphon room 4, passing through the base materialinlet 4a opening under the liquid surface 12'. In the thermal syphonroom the base material is heated by heating roll 5a to remove byevaporation the low-viscosity liquid 12 contained in the base material1.

In such a system as described above, the base material 1 continues tosupply low-viscosity liquid 12 to the base material heating device 5a,so that the base material serves as the wick of a thermal syphon. On theother hand, the thermal syphon room 4 can be filled with saturated vaporof the low-viscosity liquid after air therein is discharged by theexhaust pump 15.

Also, since the base material heating region temperature Th is greaterthan the low-viscosity liquid temperature Ts which in turn is greaterthan the cooling coil temperature Tc, the vapor pressure of thelow-viscosity liquid 12 will tend to be different in various parts inthe thermal syphon region 4'. The vapor pressure Ph in the base materialheating device is greater than the vapor pressure Pw on the surface ofthe varnish, which is greater than the vapor pressure Ps on the surfaceof the low-viscosity liquid, which is greater than the vapor pressure Pcin the cooling coil region. The low-viscosity liquid vapor 12s moves atsonic speed due to this pressure gradient, so that most of the vapor iscollected into the low-viscosity liquid storage tank 2 from the basematerial heating region 5a without allowing the vapor to enter into thevarnish storage tank 3. That is, the low-viscosity liquid vapor 12sproduced in the base material heating region is returned to thelow-viscosity liquid storage tank 2 from the thermal syphon room 4 byway of the base material inlet 4a, or liquified by the cooling coil 6ato be returned to the low-viscosity liquid storage tank 2 from thecondensate tank 8 by way of the down-flow member 8b as shown in FIG. 1.

The base material 1, after being heated in the thermal syphon room 4,contains only the saturated vapor 12s of the low-viscosity liquid. Sincethe base material 1 enters into the varnish storage region 3' throughthe liquid-sealed region of the base material outlet 4b without beingexposed to the air, there is no chance for air to enter into the basematerial 1 while the base material 1 moves from the thermal syphonregion 4' to the varnish storage region 3'.

Furthermore the saturated vapor 12s of the low-viscosity liquidcontained in the base material 1 changes into a trace of low-viscositysaturated liquid and diffuses into the varnish 13 when the base materialenters into the varnish storage region 4'. The base material 1 can thusbe impregnated evenly and adequately with the varnish 13 while the basematerial passes through the varnish storage region. Also, since most ofthe low-viscosity liquid vapor 12s generated in the thermal syphon room4 is collected into the low-viscosity liquid storage tank 2, and only anextremely small quantity of the saturated vapor is brought into thevarnish storage tank by the base material 1, the viscosity of thevarnish in the varnish storage tank 3 is scarcely affected.

The expander type guide rolls 7a and 7b cause a stretching force to actin the lateral direction in the base material as the base materialpasses over the rolls, and this stretching force ends after the basematerial has passed the rolls, so that the fiber bundles in the basematerial are stretched and then contracted laterally in thelow-viscosity liquid 12 and the varnish 13, whereby the replacement ofair with the low-viscosity liquid and impregnation of the fiber bundleswith the varnish is more efficiently accomplished. As a result, thevarnish 13 is adequately absorbed into the fiber bundles of the basematerial 1.

The present inventor conducted an experimental operation of the varnishimpregnation system described above using glass fiber cloths #7628 forelectronic base material as the base material 1. The varnish 13 had thecomposition shown in Table I. Methyl ethyl ketone, the solvent of thevarnish 13, was employed as the low-viscosity liquid 12. The temperatureconditions were: Th=150° C., Tw=30° C., Ts=20° and Tc=10° C. The resultof this experimental operation of the system indicates that the systemis capable of not only controlling the quantity of the low-viscosityliquid brought into the varnish storage tank 3 to an extremely low level(10% by volume, 0.0128% by weight) without causing any adverse effectssuch as a drop in the viscosity of the varnish in the varnish storagetank, but also impregnating the base material 1 evenly and adequatelywith varnish without leaving air bubbles in the base material 1. Thetemperature vs. vapor pressure diagram of the methyl ethyl ketone usedas the low-viscosity liquid 12 is shown in FIG. 3.

    ______________________________________                                        Composition of the Varnish                                                    ______________________________________                                        Viscous liquid                                                                             Epoxy resin     100    parts                                     Solvent      Methyl ethyl ketone                                                                           25                                                            Methylene glycol                                                                              20                                                            Dimethylformamide                                                                             15                                               Setting Agent                                                                              Dicyandiamide   3.5                                              Accelerator  Imidazole       0.1                                              ______________________________________                                    

FIG. 4 shows a second embodiment of the present invention. In thisembodiment, the varnish impregnation system comprises a low-viscosityliquid storage tank 2, a varnish storage tank 3, a thermal syphon room4, a base material heating device 5, a cooling device 6, a guide device7, a varnish impregnation control device 9 and a low-viscosity liquidheating device 21.

The side wall of the storage tanks 2 and 3 are provided with overflowdams 2a and 3a and overflow pools 2b and 3b respectively, so that thelevel of the liquid in each of the storage tanks 2 and 3 can be keptconstant at the level of the storage dams. The overflow pools 2b and 3bare designed so that liquid in the pools can be returned to the storagetanks 2 and 3 by return devices of conventional design (not shown).

The thermal syphon room 4 is connected to a bleeding tube 14, an inertgas supply tube 22 and an exhaust tube 23.

The cooling device 6 comprises a primary cooling coil 6b located in thebase material inlet 4a and above the low-viscosity liquid storage region12a, and a secondary cooling coil 6c located in the base material outlet4b above the varnish storage region 13a. The cooling capacity or thecondensing capacity of the primary cooling coil 6b should be largeenough to condense all of the vapor of the low-viscosity liquid producedwhen the base material 1 is heated by the heating roll 5a. That is, thecooling capacity or condensing capacity of coil 6b should be larger thanthe liquid evaporating capacity of heating roll 5a. On the other hand,the condensing capacity of the secondary cooling coil 6c should be lowerthan that of, or within, 10 to 20% of the capacity of the primarycooling coil 6b. The space between each of cooling coils 6b and 6c andthe wall of the room 4 is heat-insulated so that vapor of thelow-viscosity liquid will not condense on the wall as might otherwiseoccur if the wall were cooled by the coils.

A low-viscosity liquid heating device 21 is provided to heat and therebycause evaporation of the low-viscosity liquid in the low-viscosityliquid storage region 12a. Heating device 21 comprises a coil 21athrough which a heating medium may be circulated from a source notshown. The heating capacity of the heating device 21 must be largeenough to at least ensure that the quantity of low-viscosity liquidvaporized is equal to the quantity (aggregate) condensed by the coolingcoils 6b and 6c. The space between the coil 21a and the wall of thethermal syphon room 4 is heat-insulated to prevent, as far as possible,the transmission of heat from the region 12a to the wall.

A condensate collection device 8 comprises a wall 8d attached to theinterior wall of the base material outlet 4b immediately below thesecondary cooling coil 6c, and a collecting tube 8c through which thecollected condensate may flow to the low-viscosity liquid storage tank 2or, more specifically, to the low-viscosity storage region 12a. Thispermits the vapor condensed by the primary cooling coil 6c to becollected into the low-viscosity liquid storage area 12a. The vapor ofthe low-viscosity liquid condensed by the primary cooling coil 6b iscollected into the storage region 12a because it falls, under gravity,through the base material inlet 4a. The wall 8d also serves as aheat-insulating wall between the base material 1 and the secondarycooling coil 6c so that the condensate of the vapor of the low-viscosityliquid, formed when the base material is cooled by the cooling coil 6c,can be prevented from depositing on the base material 1.

Except as described above, the elements of FIG. 4 correspond to elementsof FIG. 1, previously described.

When starting the system illustrated in FIG. 4, the thermal syphon room4 should be filled with the saturated vapor of the low-viscosity liquidby the procedure described below, and the vapor pressure therein shouldbe kept within a predetermined range.

Firstly, inert gas is supplied to the thermal syphon room 4 throughsupply tube 22 to exhaust air from room 4 through exhaust tube 23.During this time, the internal pressure of the thermal syphon room 4 iskept at atmospheric pressure. The liquid levels in the low-viscosityliquid storage regions 12a and 13a are thus equal to the liquid levels12' and 13' of the storage tanks 2 and 3 as indicated by the solid linesat 12'a and 13'a. After the room 4 is filled with inert gas, the flow ofinert gas is terminated.

The heating devices 5 and 21 are then actuated to generate vapor of thelow-viscosity liquid in the thermal syphon room 4, this vaporizationtaking place primarily as a result of the heat produced by heatingdevice 21. Simultaneously, only the secondary cooling coil 6c,responsible for the partial secondary cooling capacity of the system, isactuated to condense the vapor of the low-viscosity liquid. Since theprimary cooling coil 6b having a higher cooling capacity is leftunactuated, the rate of vaporization by the two heating devices 5 and 21surpasses the rate of condensation by the secondary cooling coil 6c,thus causing an increase in the vapor pressure in the thermal syphonroom 4.

As the vapor pressure in room 4 increases, the bleeding valve 16 inbleeding tube 14 is opened and the excess vapor, with inert gas, in thethermal syphon room 4 is discharged into the low-viscosity liquidstorage tank 2 through the bleeding tube 14 to maintain the internalpressure of the room 4 at atmospheric pressure. The liquid levels 12'aand 13'a remain unchanged at the levels indicated by solid lines in FIG.4. The condensate of the vapor of the low-viscosity liquid resultingfrom the condensation by the secondary cooling 6c is collected into thecondensate pool 8a and flows back into the low-viscosity liquid storagesection 12a through collection tube 8c. This prevents the condensedvapor of the low-viscosity from falling into the vanish storage region3'.

Heating by heating devices 5 and 21 is continued with valve 16 openuntil the inert gas in thermal syphon room 4 has been discharged androom 4 contains only the vapor of the low-viscosity liquid. The valve 16is then closed.

When valve 16 is closed, continued heating causes the internal pressureof the room 4 to increase. As the internal pressure of room 4 rises, theliquid sealed levels 12'a and 13'a fall to the levels indicated by thedotted lines in FIG. 4. As the liquid level 12'a of the low-viscosityliquid pool 12a falls, the depth of immersion of the low-viscosityliquid heating coil 21a in the pool 12a, and consequently the contactarea between the heating coil and the low-viscosity liquid decreases.This causes a decrease in the rate of evaporization due to thelow-viscosity liquid heating device 21 and establishes equilibriumbetween the evaporation rate and the condensation rate of the secondarycooling coil 6c.

Subsequently, when the primary cooling coil 6b is actuated, thecondensation rate of the low-viscosity liquid vapor increases whereasthe internal pressure of the thermal syphon room 4 decreases therebycausing the liquid-sealed levels 12'a and 13'a to rise. As the liquidlevel 12'a rises, the contact area between the heating coil 21a and thelow-viscosity liquid increases to thereby increase the evaporation rateby the heating device 21. Equilibrium is thus established between theevaporation rate and the condensation rate determined by cooling coils6b and 6c so that the internal vapor pressure of the thermal syphon room4 is maintained within a predetermined range.

Under the above-described condition, the base material 1 can beimpregnated with varnish in the same manner as that of the FIG. 1embodiment by feeding the base material 1 through the impregnationsystem.

The internal vapor pressure of the thermal syphon room 4 can bemaintained constant insofar as the evaporation rate of the low-viscosityliquid 12 due to heating devices 5 and 21, and the condensation rate dueto the cooling devices 6b and 6c, is concerned. However, a change invarnish impregnation conditions, such as a change in the feeding speedof the base material, upsets equilibrium between the evaporation rateand the condensation rate, resulting in fluctuation of the internalvapor pressure in the thermal syphon room 4. The apparatus of FIG. 4 iscapable of re-establishing equilibrium after occurrence of an upsetcondition.

Any change in the internal vapor pressure of the thermal syphon room 4is accompanied by a change in the liquid level 12'a of the low-viscosityliquid pool 12a and the liquid level 13'a of the varnish pool 13a. Theevaporation rate due to the low-viscosity liquid heating device 21 isthus controlled automatically depending on changes in the liquid levels.That is, when the liquid level 12'a of the low-viscosity liquid pool 12arises due to a fall of the vapor pressure, the contact area between thelow-viscosity liquid and the low-viscosity liquid heating coil 21aincreases, as indicated by the solid line in FIG. 4. As a result, therate of evaporation increases in an amount corresponding to the drop ofthe vapor pressure to prevent the drop of the internal pressure in thethermal syphon room 4. On the other hand, a rise of the vapor pressurecauses the liquid level 12'a to drop, resulting in a decrease in thecontact area between the low-viscosity liquid 12a and the low-viscosityliquid heating coil 21a as indicated by the dotted line in FIG. 4. As aresult, the rate of evaporation due to the low-viscosity liquid heatingdevice 21 decreases to limit the rise of the vapor pressure in room 4.

As described in the above, the internal vapor pressure of the thermalsyphon room 4 can be self-controlled to remain within a certain rangeeven without a pressure controller. In the case of a conventionalsystem, a stop of the operating of the system and the resulting stop ofthe feed of the base material, or a change in an operating conditionsuch as the alteration or change of the base material feeding speed, maycause the condensation rate due to the cooling device 6 to largelysurpass the evaporation rate due to the heating of the base material tothus form a vacuum or negative pressure in the thermal syphon room 4.This may cause adverse effects such as the low-viscosity liquid 12 andthe varnish 13 being drawn up into the thermal syphon room 4 through thebase material inlet 4a and the base material outlet 4b. The systemaccording to the present invention, however, is free of such problems.

FIG. 5 shows a third embodiment of the invention. The varnishimpregnation system represented by this embodiment comprises alow-viscosity liquid storage tank 2, a varnish storage tank 3, a thermalsyphon room 4, a base material heating device 5, a cooling device 6, aguiding device 7, a condensate collection device 8, a varnishimpregnation control device 9, a low-viscosity liquid heating device 21and a condensation prevention device 31.

The wall of the thermal syphon room 4 is provided with a jacket 4cexcept on the bottom wall which is a down-flow plane 8d. An appropriateheating medium is supplied to the jacket from a suitable source (notshown) to heat and maintain the low-viscosity liquid in room 4 at atemperature higher than that of the boiling point of the low-viscosityliquid. The jacket enables the internal pressure of the thermal syphonroom 4 to be quickly raised to the saturated vapor pressure of thelow-viscosity liquid 12. At system start up, this quick rise can beaccomplished without causing condensate of the vapor 12s of thelow-viscosity liquid to deposit on the internal surface of the wall 4c.The external and bottom walls of the jacket are formed of aheat-insulating material.

The condensation prevention device 31 comprises a base material passagecylinder 32 which surrounds a base material passage region 32' extendingfrom the vicinity of the heating roll 5a to the varnish pool 13a. Thelower end of the base material passage cylinder 32 serves as the basematerial outlet 4b, and the wall of the outlet forms the heating walls32a and 32b having a jacket construction. The heating walls 32a and 32bare designed to permit a heating medium to flow therein whenevernecessary in order to heat the base material 1 to a temperature higherthan the condensation temperature of the low-viscosity liquid vapor 12sin the base material passage cylinder and the base material passageregion 32'. This system is especially characterized by being designed sothat a heating medium of a higher temperature is supplied to and flowedthrough the lower heating wall 32b surrounding the varnish pool 13a soas to heat the liquid surface 13' of the varnish in varnish pool 13 to atemperature higher than the boiling-point temperature of thelow-viscosity liquid 12. Similarly, the same heating medium supply andflow system may be applied to the upper heating wall 32a. In this case,however, the two heating walls 32a and 32b may be formed into acontinuous jacket construction.

The external walls of the jackets of the heating walls 32a and 32b areformed of heat insulating material so that heat in the base materialpassage cylinder 32 is not transmitted outside. More specifically, theheat-insulating walls are provided to prevent such adverse effects as alowering of the cooling effect of the cooling device 6 and a rise in thetemperature of the varnish 13 around the varnish pool 13a. Also, it isdesirable for the upper end of the base material passage cylinder 32 tobe located as close as possible to the heating roll 5a.

The embodiment of FIG. 5 is similar in many respects to previouslydescribed embodiment hence all of its elements are not described.However, it is noted that the condensate collection device 8 may be likethat shown in FIG. 1.

In FIG. 5, the base material 1 passes over the heating roll 5a, entersthe base material passage cylinder 32, and moves into the varnish pool13a. The inside of the base material cylinder 32 is heated to, andmaintained at, a temperature higher than the condensation temperature ofthe low-viscosity liquid vapor 12s by flowing a heating medium throughwalls 32a and 32b. The base material passage region 32' extending fromthe heating roll 5a to the varnish pool 13a is thus thoroughly filledwith the vapor of the low-viscosity liquid 12s, and the low-viscosityliquid vapor in the base material passage region 32' is prevented fromcondensing on the surface of the base material or entering into thevarnish pool 13a as might otherwise occur due to the effect of theradiational cooling of the cooling device 6.

In general, the varnish temperature is lower than that of the saturatedvapor of the low-viscosity liquid, so the low-viscosity liquid vapor 12scontained in the base material would normally condense when the basematerial 1 enters into the varnish 13. However, the liquid surface 13a'of the varnish pool 13a is heated to a temperature higher than theboiling point temperature of the low-viscosity liquid 12 by the lowerheating wall 32b, so that the condensate generated when the basematerial enters into the varnish 13 is re-evaporated so that practicallyno vapor enters the varnish 13. The varnish 13 thus remains undiluted bythe low-viscosity liquid.

In the varnish impregnation system of FIG. 5, the heating walls 32a and32b may be formed simply as heat-insulating walls which prevent the basematerial 1 passing the base material passage cylinder 32 from beingsubjected to the radiational cooling effect of the cooling device 6,while at the same time permitting the temperature of the base material 1and the ambient temperature within the region extending from the heatingroll 5a to the varnish pool 13a to be maintained at a temperature higherthan the condensation temperature of the low-viscosity liquid 12. Also,the heating wall 32a need not necessarily be cylindrical in form tosurround the base material completely but may take the form of twoopposite walls facing the sides of the descending base material.Further, the heating means at the varnish pool 13a is not necessarilylimited to the heating wall 32b with jacket construction but may takeany other conventional form.

Where there is a possibility that the vapor of the solvent contained inthe varnish 13 might enter into the thermal syphon room 4 when thevarnish pool 13a is heated, the base material outlet 4b should have thesmallest possible cross-sectional area consonant with free passage ofthe base material so that the area of the boundary between the varnishstorage region 3' and the thermal syphon region 4' is kept to a minimum.For example, as shown in FIG. 6, part of the outlet 4b, that is, theheating wall 32b, should be formed with a metal nozzle 34. The nozzle 34is provided with a fluid conduit 34a through which a heating medium mayflow to heat the varnish pool 13a to a temperature higher than theboiling point temperature of the low-viscosity liquid 12. It isdesirable that the external surface of the nozzle 34 be provided with aheat-insulating layer 35 to prevent the surrounding varnish 13 frombeing heated.

The arrangement shown in FIG. 6 prevents, to the greatest possibleextent, the evaporation of the solvent in the varnish 13 due to heatingof the varnish pool 13a, and entry of the vapor into the thermal syphonroom 4. Furthermore, condensation of the low-viscosity liquid vapor 12sat the liquid surface 13a' of the varnish pool 13a can be reduced.

As illustrated in FIG. 6, the surrounding wall of the thermal syphonroom 4 or, more particularly, the base material outlet 4b, may beprovided with a plurality of vapor injection nozzles 36 for injectinglow-viscosity liquid vapor 12s into the thermal syphon room 4. Thisarrangement, coupled with the aforementioned heating by the surroundingwall 4c of the thermal syphon room 4, permits a quick rise in vaporpressure to be realized at the start of the operation.

Further, as shown in FIG. 7, the open top of the low-viscosity liquidstorage tank 2 may be closed with a cover 2c that is provided with aninlet 2d for entry of the base material 1. The inlet 2d may be providedwith a collar 37 comprising, for example, a cooling coil in order toprevent leakage of the vapor. At the inlet 2d, the low-viscosity liquidvapor 12s is condensed or liquefied by a cooling coil 37. Thus, thediffusion of the vapor of the low-viscosity liquid is prevented, therebyreducing environmental pollution and permitting the use of a flammableliquid as the low-viscosity liquid 12. A similar arrangement may beprovided for the varnish storage tank 3.

A varnish impregnation system according to the embodiment illustrated inFIG. 8 comprises a low-viscosity liquid storage tank 2, a varnishstorage tank 3, a thermal syphon room 4, a base material heating device5, a cooling device 6, a guiding device 7, a condensate collectiondevice 8, a varnish impregnation control device 9 and a vapor flowdevice 41.

The vapor flow device 41 comprises a low-viscosity liquid storagesection 43 surrounding the base material outlet 4b and a low-viscosityliquid pool 12a provided with a self-controlled heater 44. Thelow-viscosity liquid storage section 43 is formed at the lower end ofthe base material outlet 4a and communicates with the low-viscosityliquid storage tank 2 through a passage 42a so that the low-viscosityliquid 12 can be stored in a predetermined quantity. The liquid surfacelevel of the low-viscosity storage section 42 coincides with the liquidsurface 12' of the low-viscosity liquid storage tank 2 regardless of theevaporation of the low-viscosity liquid in the storage section 42. Thelow-viscosity liquid heater 43 comprises a wall with jacket constructionsurrounding the low-viscosity liquid storage section 42. The jacketallows an appropriate heating medium to flow therein to heat andevaporate the low-viscosity liquid 12 in the low-viscosity liquidstorage section 42. The jacket wall of liquid heater 43 is made ofheat-insulating material except for those parts in contact with thelow-viscosity liquid storage section 42 and the varnish pool 13a so thatthe heat of the jacket is not transmitted to the varnish 13 around thevarnish pool 13a. The low-viscosity liquid heater 43 is formed into acontinuous jacket wall construction communicating with the surroundingwall 4c of the thermal syphon room 4.

The self-controlled heater 44 comprises a heat transfer coil throughwhich a heating medium flows from a source not shown. The heater is usedto heat and evaporate the low-viscosity liquid 12 of the low-viscosityliquid pool 12a. The evaporation capacity of the self-controlled heater44 is lower than that of the low-viscosity liquid heater 43, and heater44 is provided with a low-viscosity liquid heating plane 44a extendingboth upward and downward from the surface of the low-viscosity liquidpool 12a. The heater 44 is designed so that the evaporation capacity canbe self-controlled according to variation of the internal vapor pressureof the thermal syphon room 4, in a manner similar to that of thelow-viscosity liquid heating device 21 of FIG. 4.

In FIG. 8, the evaporation rate of the low-viscosity liquid to formvapor 12s in the thermal syphon room 4 is decreased by theself-controlled heater 44 as the vapor pressure rises in room 4, and thelow-viscosity liquid vapor 12s is caused to flow from base materialoutlet 4b toward base material heater 5a and from heater 5a towardmaterial inlet 4a, as illustrated by the arrows in FIG. 8. Thus, atleast the base material passage region extending from the base materialheater 5a to the varnish pool 13a is filled with atmosphere of thelow-viscosity liquid vapor without air. Thus, the base material may beimpregnated with varnish without air voids.

As shown in FIG. 9, the vapor flow system 41, may be provided with acooler or cooling coil 45 at the base material inlet 4a in place of theheater 44 of the preceding embodiment. The low-viscosity liquid vapor12s generated in the thermal syphon room 4 is condensed by the cooler 45and collected into the low-viscosity liquid pool 12a. This results inflow of the low-viscosity liquid vapor 12s as indicated by the arrows.In this arrangement, the bleeding tube 14 is not necessarily required.

As shown in FIG. 10, both the heater 44 of FIG. 8 and the cooler 45 ofFIG. 9 may be omitted on the side of the base material inlet 4a. Thecover 2c and the cooler 37 are provided to prevent diffusion of thelow-viscosity liquid vapor 12s, as described with reference to FIG. 7.

FIG. 11 illustrates an embodiment wherein a liquid collector 46communicating with the low-viscosity liquid storage section 42 throughthe passage 46a, is provided for collecting the condensed vapor of thelow-viscosity liquid. The collector 46 is provided with a cooler 45 tocondense and collect the condensate of the low-viscosity liquid vaporinto collector 46.

FIG. 12 illustrates an embodiment of the invention which is basicallysimilar to the embodiment of FIG. 8 but differs from each of theforegoing embodiments in that the base material inlet 4a opens above theliquid surface 12' of the low-viscosity liquid storage tank 2. The basematerial 1 is exposed to the air while moving from the low-viscosityliquid storage tank 2 into the thermal syphon room 4, but air will notenter into the base material 1 before the base material moves into thethermal syphon room 4 since the base material is impregnated with thelow-viscosity liquid. Thus, absence of the liquid seal at the basematerial inlet 4a causes no problem.

In this case, and as shown in FIG. 13, the vapor diffusion preventiondevices 2c and 37 may be provided for preventing the diffusion of thelow-viscosity liquid vapor 12s from the low-viscosity liquid storagetank 2. The effects of the cover 2c and cooling coil 37 are greater thanthat of a liquid seal at the base inlet 4a. Also, as shown in FIG. 14,instead of providing the low-viscosity liquid collector 46, thecondensate may be directly collected into the low-viscosity liquidstorage tank 2 from the base material inlet 4a.

While specific embodiments of the invention have been described indetail, obvious modifications and substitutions falling within thespirit and scope of the invention will be obvious. It is intendedtherefore to be limited only by the scope of the appended claims.

What is claimed is:
 1. A varnish impregnation apparatus comprising:alow-viscosity liquid storage tank containing a low-viscosity liquid suchas a solvent; a varnish storage tank for holding a varnish; a thermalsyphon room provided with a base material inlet opening under a liquidsurface in said low-viscosity liquid storage tank and a base materialoutlet opening under a liquid surface in said varnish storage tank; aguiding device for guiding a sheet-like base material along a pathsequentially through said low-viscosity liquid storage tank, saidthermal syphon room and said varnish storage tank; and a heating devicefor heating said base material in said thermal syphon room to evaporatelow-viscosity liquid impregnated in said base material as it is guidedthrough the low-viscosity liquid in said low-viscosity liquid storagetank to thereby produce a vapor of said low-viscosity liquid in saidthermal syphon room.
 2. A varnish impregnation apparatus as claimed inclaim 1 and further comprising a cooling device for condensing andliquefying the vapor of said low-viscosity liquid in said thermal syphonroom, and a condensate collection device for collecting said condensedand liquefied low-viscosity liquid vapor.
 3. A varnish impregnationapparatus as claimed in claim 1 and further comprising a condensationprevention device for heating said base material to a temperature higherthan the condensation temperature of said low-viscosity liquid vapor ina base material passage region extending from said base material heatingdevice to an opening of said base material outlet.
 4. A varnishimpregnation apparatus as claimed in claim 1 wherein said base materialinlet extends into the low-viscosity liquid in said low-viscosity liquidstorage tank to define a pool of said low-viscosity liquid having anupper surface which rises and falls between levels in accordance withvariations in pressure in said thermal syphon room, said apparatusfurther comprising a heating device extending into said pool whereby therate of evaporation of said low-viscosity liquid is controlled inaccordance with fluctuations in the level of said upper surface.
 5. Avarnish impregnation apparatus as claimed in claim 1 and furthercomprising a vapor flow device positioned at said base material outletand comprising a low-viscosity liquid storage section and alow-viscosity liquid heater for heating and evaporating saidlow-viscosity liquid in said low-viscosity liquid storage section, andfurther for allowing said low-viscosity liquid vapor generated in saidthermal syphon room to flow from said base material outlet to said basematerial inlet.
 6. A varnish impregnation apparatus as claimed in claim5 wherein said low-viscosity liquid storage tank and said low-viscosityliquid storage section communicate with each other.
 7. A varnishimpregnation apparatus as claimed in claim 5 wherein said vapor flowdevice is further provided with a low-viscosity liquid collectorpositioned at said base material inlet and communicating with saidlow-viscosity liquid storage section, and a cooling device provided atsaid low-viscosity liquid collector.
 8. A varnish impregnation apparatusas claimed in claim 1 wherein an outer wall of said thermal syphon roomcomprises a jacket structure so that a heat medium may be supplied toand allowed to flow in said jacket.
 9. A varnish impregnation apparatusas claimed in claim 1 which further includes a heating means for heatinga boundary area of the liquid surface of said varnish in said basematerial outlet to a temperature higher than the boiling pointtemperature of said low-viscosity liquid.
 10. A varnish impregnationapparatus as claimed in claim 9 wherein said base material outlet has anoutlet opening the cross-sectional area of which is made as small aspossible consonant with free passage of said base material therethrough.11. A varnish impregnation apparatus as claimed in claim 1 and furthercomprising a varnish impregnation control device for adjusting thequantity of said varnish impregnated in said base material after passingthrough said varnish storage tank.
 12. A varnish impregnation apparatusas claimed in claim 1 and further comprising means including a vaporinjection nozzle for supplying said low-viscosity liquid vapor into saidthermal syphon room.
 13. A varnish impregnation apparatus as claimed inclaim 1 wherein said low-viscosity liquid storage tank is closed with acover having said base material inlet, a further inlet in said coverthrough which said base material may enter said low-viscosity liquidstorage tank, and means provided at said further inlet for preventingvapor leakage from the low-viscosity liquid storage tank through saidfurther inlet.
 14. A varnish impregnation apparatus comprising:firstmeans for impregnating a sheet-like base material with a low viscosityliquid to drive from said base material air contained therein,said firstmeans including a first liquid storage tank for storing saidlow-viscosity liquid and means for feeding said base material throughsaid low-viscosity liquid in said first liquid storage tank to therebyimpregnate said base material with said low-viscosity liquid; secondmeans for vaporizing said impregnated low-viscosity liquid in said basematerial while preventing air from entering said base material,saidsecond means comprising an enclosed thermal syphon region having aninlet for receiving impregnated base material from said first liquidstorage tank, means for feeding said impregnated base material from saidfirst liquid storage tank into said thermal syphon region through saidinlet, and heater means for heating said impregnated base material insaid thermal syphon region to form in said region a saturated vapor ofthe low-viscosity liquid impregnated in said base material; and, thirdmeans for impregnating the heated base material with a varnish withoutexposing it to air, said third means comprising a second storage tankfor storing said varnish, an outlet in said thermal syphon region, saidoutlet being sealed by said varnish in said second storage tank, andmeans for feeding said heated base material from said thermal syphonregion into said varnish through said outlet.
 15. A varnish impregnationapparatus as claimed in claim 14 wherein said inlet is sealed by thelow-viscosity liquid in said first liquid storage tank.
 16. A varnishimpregnation apparatus as claimed in claim 14 wherein said inlet extendsinto the low-viscosity liquid in said first liquid storage tank todefine a pool of said low-viscosity liquid having an upper surface whichrises and falls between levels in accordance with variations in pressurein said thermal syphon region said apparatus further comprising aheating device extending into said pool whereby the rate of evaporationof said low-viscosity liquid is controlled in accordance withfluctuations in the level of said upper surface.
 17. A varnishimpregnation apparatus as claimed in claim 14 which further includes aheating means for heating a boundary area of the liquid surface of saidvarnish in said outlet to a temperature higher than the boiling pointtemperature of said low-viscosity liquid.
 18. A varnish impregnationapparatus as claimed in claim 14 further comprising means for returningto said first liquid storage tank from said thermal syphon region anylow-viscosity liquid resulting from condensing of vaporizedlow-viscosity liquid in said thermal syphon region.
 19. A varnishimpregnation apparatus as claimed in claim 18 and further comprisingcondenser means in said thermal syphon region for condensing vaporizedlow-viscosity liquid in said region.
 20. A varnish impregnationapparatus as claimed in claim 14 wherein said inlet extends into thelow-viscosity liquid in said first liquid tank to define a pool of saidlow-viscosity liquid having an upper surface which rises and fallsbetween levels in accordance with variations in pressure in said thermalsyphon region, said apparatus further comprising a heating deviceextending into said pool whereby the rate of evaporation of saidlow-viscosity liquid is controlled in accordance with fluctuations inthe level of said upper surface and further comprising condenser meansin said thermal syphon region for condensing vaporized low-viscosityliquid in said room, the capacity of said heating device being largeenough that the quantity of low-viscosity liquid it vaporizes is atleast as great as the quantity condensed by said condenser means.